This invention relates in general to color television picture tubes and in particular, to a system for implosion protecting such tubes. Conventionally, a color television picture tube has a glass bulb including a funnel, a flanged faceplate sealed to the flared end of the funnel, and an electron gun assembly mounted in the funnel neck for providing a source of cathode rays. The faceplate has a concave inner surface on which is deposited an electron-excitable phosphor screen. After the faceplate is sealed to the funnel, the glass bulb is evacuated and, as a result, a ton or more of atmospheric pressure is exerted against the external surface of the faceplate. A glass bulb of this type is subject to implosion. The term "implosion" is defined by Underwriters Laboratory Incorporated as a "rapid and sudden inward bursting of a high-vacuum glass envelope." It is of the utmost importance in the interest of safety to prevent the faceplate from violently shattering should it be struck for example, by a heavy missile, for when a bulb implodes, fragments of glass may fly forwardly from the tube toward the viewer.
Three basic approaches for implosion protecting color cathode ray tubes (CRT's) have evolved. These three approaches employ different principles of operation. One approach is implemented in systems referred to as "rimbond" system. The rimbond system has a scalloped metal frame which surrounds the flange found on every conventional faceplate. The gap between the frame and the faceplate flange is filled with cement--typically an epoxy cement. In a rimbond system, the frame is not under tension. The cement holds in position the pieces of glass of a shattered faceplate long enough for air to enter the tube through the cracks formed so that pressure builds up in the tube relatively slowly. This prevents unacceptable amounts of glass from being projected forwardly from the tube, although the tube may still collapse. Patents illustrating such rimbond systems are U.S. Pat. Nos. 3,485,407; 3,558,818; 3,412,203 and 3,835,250. A major drawback to such rimbond systems has been the large amounts of very costly epoxy cement needed to adhere the metal frame to the faceplate.
A second basic implosion protection approach is termed the "tension band" approach. Systems implementing this approach comprise a strap or band which is placed around the faceplate flange and put under very high tensile force. Numerous patents have been issued on various aspects of tension band systems. See U.S. Pat. Nos. 3,818,557; 3,777,057; 3,845,530; and 3,890,464. The tension band systems, however, also have several drawbacks. When the tension band is tightened about the faceplate flange, it is very likely that the glass will be scratched as the band moves across it during the tightening process. This creates flaws at the location of the scratches, increasing the possibility of cracks forming there during implosion. Also, the distribution of forces applied to the faceplate flange by the band is irregular. Specifically, the forces applied at the corners by the band are much greater than the forces applied at the sides of the faceplate flange. In some of the tension band systems, an adhesive material is used under the tension band before tightening. The drawback here, however, is that as the tension band is tightened, there is a tendency to press away the adhesive material.
A third basic system is the face shield system (commonly referred to as "bonded panel") which was the original system developed to protect color television tubes from implosion. This system consists of a section of transparent plate glass having the size, shape and dished contour of the front surface of the faceplate of the picture tube. The plate glass is bonded to the faceplate with a clear resin. Since the television picture must be viewed through the face shield system, optical clarity is imperative. Many rejects occur due to bubbles forming in the resin which must be poured in between the faceplate and the shield. It is a prohibitively expensive system in the present picture tube market. Patents disclosing such systems are U.S. Pat. Nos. 3,315,035; 3,321,099; and 3,708,622.
A U.S. Pat. No. 2,222,197 to Engels discloses a CRT in which the CRT envelope comprises a curved, flangeless, faceplate inset in an expanded open end of a cooperating funnel. A band allegedly providing implosion protection surrounds the funnel near the open end thereof in a plane intersecting the faceplate ensconsed within the funnel mouth. The Braun system is quite different from the present system. No frame of any sort is provided. The implosion band environs the funnel rather than the faceplate peripheral edge. A comparison of the Engels' system and present system will reveal other important differences also.
This invention is believed to be most useful when applied to a unique tube having a flangeless faceplate. Such a tube is disclosed in U.S. Pat. No. 3,894,260, issued to the assignee of this application. The tube has a flangeless, curved, glass faceplate, a concave inner surface of which receives a phosphor screen. The funnel portion of this unique tube has a convex seal land which matches and mates with the curvature of the concave inner surface of the faceplate. Since the faceplate is flangeless, the sealing interface between the funnel and faceplate is curved rather than planar as in conventional tubes. Conventional rimbond and tension band system structures, as described above for implosion protection, would not appear to be readily adaptable for use with this tube.